The invention relates to an optical information medium for erasable high-speed recording by means of a laser-light beam, said medium comprising a substrate carrying a stack of layers, which stack comprises, in this order, a first dielectric layer, a recording layer of a phase-change material comprising a compound consisting of Ge, Sb, Te and O, a second dielectric layer and a metal mirror layer.
The invention also relates to the use of such an optical recording medium in high storage density and high data rate applications.
Optical information or data storage based on the phase change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a thin crystalline film using a focused laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam which is modulated in accordance with the information to be recorded. Due to this, quenching takes place in the phase-change recording layer and causes the formation of amorphous information bits in the exposed areas of the recording layer which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by recrystallizing through heating with the same laser. The amorphous marks represent the data bits, which can be reproduced via the substrate by a low-power focused laser-light beam. Reflection differences of the amorphous marks with respect to the crystalline recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information.
One of the main problems in high speed phase-change optical recording is the required erasing (recrystallization) speed. High crystallization speed is particularly required in high-density recording and high data rate applications, such as disc-shaped DVD-RAM and optical tape, where the complete crystallization time (complete erase time: CET) has to be shorter than 50 ns. If the crystallization speed is not high enough to match the linear velocity of the medium relative to the laser-light beam, the old data (amorphous marks) from the previous recording cannot be completely removed (recrystallized) during DOW. This will cause a high noise level.
An optical information medium of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,100,700. The known medium of the phase-change type comprises a substrate carrying a stack of layers consisting, in succession, of a first dielectric or protective layer, a recording layer of a phase-change Ge-Sb-Te compound comprising between 5 and 20 at. % oxygen, a second dielectric layer and a metal reflective layer. Such a stack of layers can be referred to as an (S)IPIM-structure, wherein S represents a substrate, M represents a reflective or mirror layer, I represents a dielectric layer and P represents a phase-change recording layer. Said patent does not disclose any values about CET or erasing speed.
As will be shown below, such relatively high oxygen concentrations in the recording layer adversely affect not only the CET-value, but also other important parameters, such as jitter and cyclability. Cyclability is expressed as the maximum number of repeated writing (amorphization) and erasing (recrystallization) operations wherein e.g. jitter or change in reflectance of the recording medium remains below a specified value.